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[Towertalk] Co-ax Traps

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Subject: [Towertalk] Co-ax Traps
From: (Chuck Counselman)
Date: Wed, 25 Sep 2002 13:14:25 -0400
At 6:52 PM +0200 9/25/02, Maurizio Panicara wrote:
>No, it is not important.

I must disagree.  It is essential!  The trap is a parallel-resonant, 
nearly lumped, LC circuit.  The C stems mainly from the capacitance 
between the center conductor and the shield/braid of the coax.

>Although easy to build, coax traps have more losses than conventional ones
>and don't handle much power.

It's not a bad way to construct a trap, as long as you seal the ends 
of the coax to keep water out.  A little water will do a lot of harm, 
by corroding the braid and thereby causing a lot of loss.

The polyethylene dielectric of coax makes a good high-Q capacitor. 
The large-diameter braid of coax makes a low-resistance conductor, 
which makes for a high-Q inductor.

If you have NEC-2 or NEC-4 or a commercial variant, use it to 
calculate the voltage across the trap when the antenna is driven at 
the resonant freq. of the trap, to check that the breakdown voltage 
of the coax is not exceeded.  IIRC, RG-8-size coax handles 5 kV rms, 
but I don't know about RG-58-size.  Also check the RF current against 
the limit for the given coax.  Heating of the center conductor 
softens the dielectric and leads to shorting.

Peter Larsen pointed out a website 
where you can download a program for designing a trap with 
practically any kind of coax.

73 de Chuck, W1HIS

>----- Original Message -----
>From: "Peter Larsen" <>
>To: <>
>Sent: Wednesday, September 25, 2002 5:50 AM
>Subject: [Towertalk] Co-ax Traps
>>  I am thinking of building a set of co-ax traps to make a 40/80/160
>>  inverted V.
>>  I noticed in the ARRL antenna hand book that the braid is the input
>>  and the center conductor is the out put.
>  > Is this important, and why?

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